This invention relates to plastic bags and to methods of filling them.
The invention is concerned with such plastic bags which are used as liners (hereinafter called xe2x80x9cliner bagsxe2x80x9d) for bulk bags for containing free flowing moisture sensitive granular material. Hitherto such liner bags were used especially but not exclusively to line one tonne bulk bags for sugar. The bulk bags are normally woven plastics bags of sufficient strength to carry such content and when filled with free flowing granular material take up a cuboidal form. The upper surface of the bulk bag is open and a closure flap means is provided which span this open surface when the bulk bag has been filled to provide a closure therefor. Loops are provided at the corners of the bulk bags so that they may be lifted by the forks of a fork lift truck. The liner bag is used to contain the granular material and to protect the contents against ingress of contaminants and of air containing moisture and consequent formation of lumps of sugar because of the water content of the air.
In known arrangements the liner bag is formed from blown plastic tube normally formed with deep gussets so as to be able to take up the cuboidal shape. In the lie flat condition, the liner bag comprises a front panel and a rear panel and each gusset comprises a front gusset part and a rear gusset part, the side edges of the front panel being joined to the front gusset parts and the side edges rear panel being joined to the rear gusset parts. The liner is longer than the bulk bag and is of constant width. Its upper end is not sealed. The liner bag is inserted into the bulk bag and its upper end is opened to receive the filler nozzle through which sugar is delivered into the liner bag. In many cases initially a blast of air causes the liner bag to expand whereafter the granular material is fed into the liner bag through the nozzle. After filling the liner bag, in most cases, is vibrated causing the sugar settle to a maximum level. The upper portion of the liner bag is wound around and twisted after which it is wire tied and knotted or taped to seal it. To transport the material, the bulk bag is lifted by the loops and the upper portion of the liner bag may also form a retaining part that is also secured to the lifting device. The base of the bulk bag has an opening therein normally closed by a bottom flap. To discharge the contents of the bulk bag, it is located above the material receptacle. The top opening is opened, the wire ties and tape are removed and the knot undone. The bottom flap is opened and the liner bag is pierced so that the sugar will flow out of the liner bag.
Such a system is widely used in the sugar industry. It does however suffer from a number of disadvantages. First, the knot (together with the wire ties and the adhesive tape) often does not provide an adequate seal so that the sugar may be contacted by the water content in the ambient air so that it becomes lumpy. Second, it is relatively easy to undo and re-tie the wire ties and the knot so that the bulk bags are liable to pilferage which cannot be easily visually detected.
U.S. Pat. No. 4,341,054 (Courtheoux) discloses heat welding the mouth of a plastic bag. The bag is small, being designed to accommodate 2500 gm of liquid syrup and is not gusseted. Thus there are no undue technical difficulties in heat welding the bag. However it is not possible to adapt this teaching to liner bags for bulk bags because the length of the required heat weld would be of the order of two metres and the liner bags would be deeply gusseted. In practice it has hitherto been extremely difficult to place the various panels of the liner bag flat and close together and consequently there will be folds in the plastic sheeting which will prevent the formation of a satisfactory moisture proof sealing of the plastic parts. In particular it would not be possible to do so speedily as would be required for normal commercial operation.
U.S. Pat. Nos. 4,596,040 and 4,781,475 (La Fleur et al) disclose a liner for a bulk bag. The bulk bag is of a particular shape with a nozzle or throat at its upper end and the liner bag is shaped to correspond to the shape of the bulk bag. The liner bag is heavily gusseted to take up the cuboidal shape and has a reduced dimensioned neck into which the gusset extends. The nozzle is closed by being tied off by a cord. This suffers from the disadvantages of inadequate sealing and liability to pilferage as mentioned above.
According to one aspect of the invention there is provided a liner bag comprising a front panel, a rear panel and gussets between the sides of the panels, each gusset comprising a front gusset part and a rear gusset part, the side edges of the front panel being joined to the front gusset parts and the side edges of the rear panel being joined to the rear gusset parts, the liner bag comprising a body part and a neck at its upper end, which neck is adapted to receive therein a filling nozzle and which neck can be heat sealed after the liner bag has been filled to seal liner bag, the neck being formed by welding together the edges of the front panel to the front gusset parts and by welded together the edges of the rear panel to the rear of gusset parts; wherein the front and rear panels and the gusset parts are all welded together only at the free end of the neck.
According to another aspect of the invention there is provided a method of manufacturing a liner bag as set out in the preceding paragraph comprising forming a gusseted lay flat tube; passing the tube over a welding table; by means of a welding unit, welding the tube to form the neck; and interposing blanking pieces between the front and rear parts of the gussets to prevent the front panel and front gusset parts from being welded to the rear panel and rear gusset part and locating such blanking pieces so that they, the blanking pieces, are spaced from the end of the neck with the result that the front panel, the front gusset parts, the rear gusset parts and the rear panel are welded together at such spaced locations.
According to a further aspect of the invention there is provided a method of filling a bulk bag with free flowing material e.g. sugar, in which a liner bag, as set out in the preceding paragraph but one, is inserted into the bulk bag, the neck is opened and placed around the filler nozzle of a bulk filling machine, and the liner bag is filled, wherein inward pressure is then applied between the gusset parts to re-form the gussets and cause the panels and gusset parts to lie flat against one another and then heat sealing the panels and the gusset parts at the part of the neck near the body part to seal the liner bag closed.
According to a further aspect or the invention there is provided the combination of a bulk bag containing therein a liner bag of the invention as set forth above, the bulk bag having an open top and flap means which closes the open top when the bag is filled.
Embodiments of the invention will now be described by way of example with reference to the accompanying drawings.